The power analysis in MetaboAnalyst is based on R package SSPA described here

The calculation is based on two assumptions:

- The test statistics follow a normal or near normal (Students’ t) distribution.
- The effect is indeed present in the data;

The diagnostic plot is to help assess whether these two assumptions are reasonably met. In particular, the panels on the left (**normality assessment**) shows the distribution of the test statistics (t-stat) as a histogram (*top panel*) and as QQ plot (*bottom panel*); the panels on the right shows the distribution of the raw p values as histogram (*top panel*) and the bottom panel shows the sorted p values against their ranks (**effect assessment**). We expect a large proportion of p values will be close to zero (indicating strong effect) - **i.e.** the histogram should be left-shifted and the bottom graph should shift to top left corner.